High-power redox flow battery based on the CrIII/CrVI redox couple and its mediated regeneration

ABSTRACT

This invention describes a high-power, high-energy oxidant (catholyte) chemistry based on dichromate (Cr VI  as Cr 2 O 7   2− ) for use with a variety of fuels (anolytes) in redox flow batteries (RFBs, also known as reversible fuel cells), which reversibly store electricity as chemical energy. The reduction (discharge) of Cr 2 O 7   2−  to Cr 3+  is natively irreversible at all investigated solid-state electrocatalysts, which has historically limited the employment of Cr 2 O 7   2−  to primary (non-rechargeable) cells, such as Grenet cells. The described invention overcomes this limitation by using a reversible redox couple, hereafter electron mediator, to heterogeneously donate electrons to the cathode electrocatalyst and homogeneously accept electrons from Cr 3+  to regenerate Cr 2 O 7   2− . RFBs employing this energy- and power-dense chemistry are suitable for low-cost energy storage applications, ranging from grid-level storage of renewable electricity to consumer electronics.

CROSS REFERENCE TO RELATED APPLICATION

This present application is a U. S. national stage filing under 35U.S.C. § 371 of International Application No. PCT/EP2016/063943, filedJun. 16, 2016, which claims priority to EP Patent Application Serial No.15020098.8, filed Jun. 16, 2015 the disclosures of which are hereinincorporated by reference in-their entireties.

BACKGROUND OF THE INVENTION Field of the Invention

The invention describes novel redox flow battery systems, which areelectrochemical cells used for electricity storage.

Description of the Related Art

Redox flow batteries (RFB) are secondary (rechargeable) fuel cells andbattery-fuel cell hybrids (FIG. 1). Unlike traditional batteries, RFBsdecouple system energy and power, and so like a fuel cell have a totalsystem energy that scales with the size of electroactive materialstorage tanks and a system power that scales with the size of theelectrochemical reactor. This trait makes them ideal for storing largeamounts of electricity at low cost, since additional electroactivematerial is usually far less expensive than a larger reactor. For thisreason, RFBs are currently used and being further developed as massive,grid-level electricity storage devices (J. Power Sources 2006, 160,716-732). This is especially valuable for intermittent renewable energysources, which, in order to stabilize their output to current electricalgrids, require a source of backup electricity when the given resource(wind, solar) becomes temporarily unavailable.

Many RFB chemistries have been developed, yet they universally useoxidants that undergo 1 or 2 e⁻ reductions (n=1 or 2) and/or have low tomoderate solubility (C_(max)), greatly limiting (1) their volumetricenergy densities and (2) their power densities. This applies to VO₂ ⁺,Ce⁴⁺, Fe³⁺ (1 e⁻), Br₂ (2 e⁻), and O₂ (4 e⁻), the oxidants inall-vanadium, Zn/Ce⁴⁺, Cr²⁺/Fe³⁺, H₂/Br₂, and H₂/O₂ RFBs, respectively.While O₂ and Br₂ have high n, their values for C_(max) are much smallerthan those for the other species (0.001 and 0.2 M vs.>1 M). The lowerenergy and power densities associated with RFBs result in very largebattery sizes relative to competing technologies, such as lead-acid andLi-ion batteries. The sheer size of such batteries contributes tounwanted facilities, operational, and materials costs, as well asrestrictions on battery siting.

Thus there exists a need for oxidants of significantly higher energy andpower density. While multiple other oxidants have higher n and C_(max),such as permanganate (MnO₄ ⁻, n=5, C_(max)=7.3 M), multi-e⁻ reactions bytheir very nature tend to be irreversible and therefore unsuitable forRFBs. Many such reactions follow intricate mechanisms and result inprecipitation or poisoning at the cathode, decreasing current and poweroutput quickly over time. A reversible oxidant system with high n andC_(max) and a non-fouling reaction is unknown in the field.

The oxidant Cr₂O₇ ²⁻ is attractive for use in RFBs since its energydensity parameters (n=6, C_(max)=7.1) are similar to those of MnO₄ ⁻.Additionally, Cr₂O₇ ²⁻'s diffusion coefficient is significantly higherthan that for VO₂ ⁺ or Ce⁴⁺ (0.96 vs. 0.25 and 0.36×10⁻⁵ cm²/s),resulting in faster Cr₂O₇ ²⁻ transport and correspondingly higher powerdensity. These impressive traits of high energy and power densities ledresearchers to develop primary (non-rechargeable) Zn/Cr₂O₇ ²⁻ cells from1841 to 1859, after which they became a standard power source fortelegraphs.

RFBs based on the reduction of Cr₂O₇ ²⁻ to Cr³⁺ have for example beenproposed in FR7917793, but such systems are in fact far too inefficientfor practical use. These RFBs regenerate Cr₂O₇ ²⁻ by oxidizing Cr³⁺ atmultiple solid-state electrode catalysts, all of which oxidize Cr³⁺ atsuch high overpotentials that they result in O₂ coevolution. Suchcatalysts include Pt-group metals, poor metal oxides (e.g. PbO₂), andconductive carbon materials. The high overpotential for Cr³⁺ oxidationcauses both significant loss of voltage to heat and significant loss ofcurrent to O₂ evolution instead of Cr₂O₇ ²⁻ regeneration, resulting in asystem with low to negative recharge efficiency. For this reason, theuse of these solid-state catalysts to oxidize Cr³⁺ is exclusivelyconfined to energy-consuming (rather than energy-storing) industrial useto regenerate Cr₂O₇ ²⁻ in spent chromium plating solutions. Indeed, thepreviously proposed RFBs using Cr₂O₇ ²⁻ are so ineffective that they areeither not discussed in modern reviews of RFBs or are discussed only inthe context of their poor functionality. An energy-efficient method toregenerate Cr₂O₇ ²⁻ has not been devised in the 175 years since Zn/Cr₂O₇²⁻ primary cells were first invented, despite significant efforts overthe past 67 years. The impossibility of energy-efficient 6 e oxidationof 2 Cr³⁺ to Cr₂O₇ ²⁻ is currently so widely accepted that modern flowbattery researchers have devised low-solubility Cr complexes thatundergo sequential 1 e⁻ oxidations, resulting in significant compromiseof both the energy density and voltage available from Cr₂O₇ ²⁻.

The present invention takes a known high energy, high power,irreversibly reduced (discharged) oxidant, Cr₂O₇ ²⁻, and allows for itsreversible use in RFBs. Cr₂O₇ ²⁻'s reduction product, Cr³⁺, cannot bereadily oxidized (i.e. recharged) heterogeneously at known electrodecatalysts. Thus previous batteries using Cr₂O₇ ²⁻ served only as primary(non-rechargeable) cells. In the present invention, Cr₂O₇ ²⁻'sirreversible reaction is overcome by employing an electrochemicallyreversible electron mediator that is heterogeneously oxidized by thecathode (during recharge) and that homogeneously oxidizes Cr³⁺ back toCr₂O₇ ²⁻, completing the recharge cycle using an EC_(cat) mechanism.

Electron mediators have been described in the prior art, for example inWO 2013/131838, which uses the 1 e⁻ mediator Ce⁴⁺ to shuttle e⁻ back andforth between a catalytic bed (cf. Summary, and paras. [0008], [0009]).However, the solution presented in WO 2013/131838 specifically requiresa catalyst in addition to Ce⁴⁺ in order to perform a 4 e⁻ oxidation ofH₂O or an oxidation of other compounds.

Furthermore, it is demonstrated herein that Cr₂O₇ ²⁻ may be steadilyreduced (discharged) for extended periods of time without anyundesirable side reactions. In contrast, the reduction of MnO₄ ⁻, anoxidant with a similar energy and power density, results in rapidprecipitation of MnO₂, resulting, amongst other things, in electrodefouling.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a rechargeable redoxflow battery (RFB) comprising an electrochemical cell comprising atleast one positive electrode in a positive half-cell and a negativeelectrode in a negative half-cell, an ion-conducting membrane betweenthe two half-cells, wherein the membrane is designed for dual acidicanolyte and catholyte or designed for an acidic catholyte and analkaline anolyte; at least two storage tanks for catholyte and anolyte,one or more pumps to circulate stored catholyte and anolyte through thecathodic and anodic half cells, respectively, and at least one anolyteand at least one catholyte;

wherein

the anolyte comprises at least one fuel capable of reversible oxidation,at least one electrolyte for conductivity and at least one solvent; and

the catholyte comprises the Cr^(III)/Cr^(VI) redox couple, at least oneelectrolyte for conductivity, at least one solvent and at least oneelectrochemically reversible electron mediator; and further

wherein

said electron mediator is capable of homogeneously oxidizing Cr^(III) toCr^(VI) using an EC_(cat) mechanism in solution.

In another aspect, the present invention also relates to the use of theredox flow battery as described herein to store electrical energy forgrid-level energy storage, homeowner energy storage, remote locations,firming or load leveling of intermittent renewable electricitygeneration site, preferably wind and solar farms, micro-hydropower,geothermal energy, tidal power, energy arbitrage, portable and/orpersonal electronics, electric vehicles such as ships, submarines,planes, unmanned underwater vehicles (UUVs) or unmanned aerial vehicles(UAVs), military electronics equipment, satellites and other manned orunmanned spacecraft, or other applications where rechargeable RFBs canbe beneficially employed.

A further aspect of the present invention is a method for storingelectrical energy comprising:

-   -   a) circulating an electrolyte through a positive half-cell in        the redox flow battery as described herein,    -   b) supplying a current power source to the positive electrode of        said redox flow battery,    -   c) using an electrochemically reversible electron mediator that        is heterogeneously oxidized by the cathode to homogeneously        oxidize Cr^(III) to Cr^(VI) without concurrently oxidizing        water.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Schematic illustration of a redox flow battery comprising (1)anolyte storage tank; (2) catholyte storage tank; (3) anolyte chamber;(4) catholyte chamber; (5) anode; (6) cathode; (7) selectively permeableion-conducting membrane; (8) pump for anolyte; and (9) pump forcatholyte.

FIG. 2: RDE voltammetry of 100 mM VO₂ ⁺ (dash-dot), Ce⁴⁺ (dashed), andCr₂O₇ ²⁻ (solid), at Au in 0.5 M H₂SO₄ at 1000 rpm, 50 mV/s.

FIG. 3: Volumetric capacity of saturated solutions of Cr₂O₇ ²⁻ (7.1 M),Ce⁴⁺ (2.5 M), and VO₂ ⁺ (3 M).

FIG. 4: Table of half-wave potentials (E_(1/2)) of various Cr₂O₇ ²⁻catalysts, taken from RDE voltammetry of 5 mM Cr₂O₇ ²⁻ in 0.5 M H₂SO₄ at3000 rpm, 50 mV/s.

FIG. 5: Cyclic voltammetry (CV) of (1) a mixed solution of 5 mM Cr₂O₇ ²⁻and 5 mM Ce⁴⁺ (solid), and (2) a single-species solution of 5 mM Ce⁴⁺(dash-dot) both at Au in 0.5 M H₂SO₄ at 50 mV/s.

FIG. 6: Cyclic voltammetry (CV) of (1) a mixed solution of 5 mM Cr₂O₇ ²⁻and 5 mM Ce⁴⁺ (solid); (2) 5 mM Ce⁴⁺ (dash-dot); (3) 5 mM Cr₂O₇ ²⁻(dotted); (4) electrolyte only (dash-double-dotted); and (5) expectedcurrent from a mixed solution of 5 mM Cr₂O₇ ²⁻ and 5 mM Ce⁴⁺ whenfactoring in a background correction for Au (dashed), all at Au in 0.5 MH₂SO₄ at 50 mV/s.

FIG. 7: RDE voltammetry of a mixed solution of 2.5 mM Cr₂O₇ ²⁻ and 2.5mM Ce⁴⁺ showing (1) original current (solid with triangle); (2) currentafter a 2.5 C discharge (dashed); and (3) current following a 2.5 Crecharge (solid with circle), all at Au in 0.5 M H₂SO₄ at 250 rpm, 50mV/s.

FIG. 8A: Chronocoulometry during discharge of (1) 2.5 mM Ce⁴⁺ (dashed)and (2) 2.5 mM 2.5 mM Cr₂O₇ ²⁻ and 2.5 mM Ce⁴⁺ (solid), both at Au in0.5 M H₂SO₄ at 4000 rpm.

FIG. 8B: Chronocoulometry during recharge of 2.5 mM Cr₂O₇ ²⁻ and 2.5 mMCe⁴⁺ at Au in 0.5 M H₂SO₄ at 4000 rpm. Inset: Comparativechronocoulometry during recharge of (1) 2.5 mM Ce⁴⁺ (dashed) and (2) 2.5mM 2.5 mM Cr₂O₇ ²⁻ and 2.5 mM Ce⁴⁺ (solid), both at Au in 0.5 M H₂SO₄ at4000 rpm.

FIG. 9: RDE voltammetry of 5 mM Cr₂O₇ ²⁻ showing the anodic (solid) andcathodic (dashed) scans at Au in 0.5 M H₂SO₄ at 1000 rpm, 50 mV/s.

DETAILED DESCRIPTION

An “EC_(cat) mechanism” (also denoted as EC^(I) in Bard and Faulkner in“Electrochemical Methods: Fundamentals and Applications”, 2^(nd) Ed.2001) is described in the following general manner: it consists of anelectrochemical step (hereafter E) in which a given species A isconverted to species B. This is followed by a subsequentchemical-catalytic step (hereafter C_(cat)) between B and species C,which regenerates species A from B and produces by-product D from C. Theregeneration of A represents a catalytic process that gives an apparentincrease in concentration of A near the electrode surface, generatinghigher than expected electrochemical current for the reduction of A toB:E:A+1e ⁻ →BC _(cat) :B+C→A+D

Since regenerated A must remain at the electrode surface to be detected,high currents are detected at (1) shorter time periods betweenconsumption and regeneration of A and (2) decreased flow rates ofsolution across the electrode, which decrease the time for transport andthe rate of transport for A leaving the electrode surface.

The observed “electrochemical potential,” or simply “potential,” E, of agiven soluble redox species (e.g., Ce⁴⁺) participating in a simplistic,reversible reduction (e.g. Ce⁴⁺+1 e⁻⇄Ce³⁺) is defined by the Nernstequation:

$E = {E^{0} + {\frac{RT}{nF}\ln\frac{C_{ox}}{C_{red}}}}$where C_(ox) and C_(red) are the respective concentrations of theoxidized and reduced forms of the given redox species (e.g. Ce⁴⁺ andCe³⁺), n is the number of e⁻ involved in the conversion of the oxidizedto the reduced form of the given redox species, R is the universal gasconstant, T is the temperature, and F is Faradays constant. The term E⁰is the standard potential of the redox species, which is the observedpotential (E) when C_(ox)=C_(red). In the context of the invention, theNernst equation implies that the potential of a given reversible redoxspecies varies from its E⁰ depending on the ratio of the oxidized toreduced form present in solution. Thus a ratio of Ce⁴⁺ to Ce³⁺ of 99 to1 shifts the E of Ce⁴⁺ positive by 0.120 V, and a ratio of 99.96 to 0.04shifts E positive by 0.200 V.

A “rotating disk electrode,” RDE, is used in RDE voltammetry to achievelaminar flow across an electrode surface. When planar RDEs are used, asin the results presented herein, RDE voltammetry allows directassessment of a multitude of fundamental parameters of a given fuel oroxidant for use in RFBs. Because RDE voltammetry assesses fundamentalparameters, rather than a given fuel cell's or RFB's performance, RDEvoltammetric results are universally comparable across all possiblesystems.

The “mass transport limited current,” i_(L), of an electrochemicalreaction in RDE voltammetry is the maximum obtainable current at a givenRDE rotation rate. Since the i_(L) is a function of the steady-statetransport that originates from laminar flow across the RDE, the i_(L) isindependent of potential, and appears as a horizontal line in an RDEvoltammogram.

The half-wave potential, E_(1/2), of a given electrochemical reaction inRDE voltammetry is the potential at which ½ of the i_(L) is achieved. Areasonably accurate measurement of E_(1/2) must be performed at fairlylow current density; at high current density in analytical glassware,solution resistance distorts the RDE voltammogram with a diagonal linethat significantly shifts the E_(1/2). To achieve low current densityfor a high n oxidant like Cr₂O₇ ²⁻, low concentration (1 to 10 mM) andlow to moderate rotation rate (50 to 3000 rpm) must be used. In thecontext of the invention, the E_(1/2) is useful as a first-orderassessment of the operating potential of a given electrode, and thus theoperating voltage of a RFB.

The terms “NHE” and “Ag/AgCl” refer to two standard types of referenceelectrodes used to measure the E of an electrode, where NHE is theNormal Hydrogen Electrode and Ag/AgCl is the silver/silver chlorideelectrode, and a given E_(Ag/AgCl)=E_(NHE)−0.197 V. Unless statedotherwise, all unreferenced potentials in this section are shown as Vvs. Ag/AgCl, not V vs. NHE.

The term storage tank refers to vessels wherein one or more liquidsand/or gases can be stored. The liquids and/or gases may be separatedfrom each other by a baffle or other suitable partition walls.

“Alkyl” refers to a straight or branched hydrocarbon chain radicalconsisting solely of carbon and hydrogen atoms, containing nounsaturation, having from one to ten carbon atoms (e.g., (C1-10) alkylor C1-10 alkyl). Whenever it appears herein, a numerical range such as“1 to 10” refers to each integer in the given range—e.g., “1 to 10carbon atoms” means that the alkyl group may consist of 1 carbon atom, 2carbon atoms, 3 carbon atoms, etc., up to and including 10 carbon atoms,although the definition is also intended to cover the occurrence of theterm “alkyl” where no numerical range is specifically designated.Typical alkyl groups include, but are in no way limited to, methyl,ethyl, propyl, isopropyl, n-butyl, iso-butyl, sec-butyl isobutyl,tertiary butyl, pentyl, isopentyl, neopentyl, hexyl, septyl, octyl,nonyl and decyl. The alkyl moiety may be attached to the rest of themolecule by a single bond, such as for example, methyl (Me), ethyl (Et),n-propyl (Pr), 1-methylethyl (iso-propyl), n-butyl, n-pentyl,1,1-dimethylethyl (t-butyl) and 3-methylhexyl. Unless stated otherwisespecifically in the specification, an alkyl group is optionallysubstituted by one or more of substituents which are independentlyalkyl, aryl, heteroaryl, hydroxy, halo, cyano, trifluoromethyl,trifluoromethoxy, nitro, trimethylsilanyl, —OR^(a), —SR^(a),—OC(O)—R^(a), —N(R^(a))2, —C(O)R^(a), —C(O)OR^(a), —OC(O)N(R^(a))₂,—C(O)N(R^(a))₂, —N(R^(a))C(O)OR^(a), —N(R^(a))C(O)R^(a),—N(R^(a))C(O)N(R^(a))₂, N(R^(a))C(NR^(a))N(R^(a))₂,—N(R^(a))S(O)_(t)R^(a) (where t is 1 or 2), —S(O)_(t)OR^(a) (where t is1 or 2), —S(O)_(t)N(R^(a))₂ (where t is 1 or 2), or PO₃(R^(a))₂ whereeach R^(a) is independently hydrogen, alkyl, fluoroalkyl, aryl, orheteroaryl.

The term “alkoxy” refers to the group —O-alkyl, including from 1 to 8carbon atoms of a straight, branched, cyclic configuration andcombinations thereof (e.g. as outlined above for alkyl) attached to theparent structure through an oxygen. Examples include, but are notlimited to, methoxy, ethoxy, propoxy, isopropoxy, cyclopropyloxy andcyclohexyloxy. “Lower alkoxy” refers to alkoxy groups containing one tosix carbons.

The term “substituted alkoxy” refers to alkoxy wherein the alkylconstituent is substituted (i.e., —O-(substituted alkyl)). Unless statedotherwise specifically in the specification, the alkyl moiety of analkoxy group is optionally substituted by one or more substituents whichindependently are: alkyl, heteroalkyl, alkenyl, alkynyl, cycloalkyl,heterocycloalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, hydroxy,halo, cyano, trifluoromethyl, trifluoromethoxy, nitro, trimethylsilany1,—OR^(a), —SR^(a), —OC(O)—R^(a), —N(R^(a))2, —C(O)R^(a), —C(O)OR^(a),—OC(O)N(R^(a))₂, —C(O)N(R^(a))₂, —N(R^(a))C(O)OR^(a),—N(R^(a))C(O)R^(a), —N(R^(a))C(O)N(R^(a))₂, N(R^(a))C(NR^(a))N(R^(a))₂,—N(R^(a))S(O)_(t)R^(a) (where t is 1 or 2), —S(O)_(t)OR^(a) (where t is1 or 2), —S(O)_(t)N(R^(a))₂ (where t is 1 or 2), or PO₃(R^(a))₂ whereeach R^(a) is independently hydrogen, fluoro, alkyl, fluoroalkyl, aryl,or heteroaryl.

The term “alkoxycarbonyl” refers to a group of the formula(alkoxy)(C═O)— attached through the carbonyl carbon wherein the alkoxygroup has the indicated number of carbon atoms. Thus a (C₁₋₆)alkoxycarbonyl group is an alkoxy group having from 1 to 6 carbon atomsattached through its oxygen to a carbonyl linker. “Lower alkoxycarbonyl”refers to an alkoxycarbonyl group wherein the alkoxy group is a loweralkoxy group.

The term “carboxy” refers to a —(C═O)OH radical.

The term “aromatic” or “aryl” or “Ar” refers to an aromatic radical withsix to ten ring atoms (e.g., C6-C10 aromatic or C6-C10 aryl) which hasat least one ring having a conjugated pi electron system which iscarbocyclic (e.g., phenyl, fluorenyl, and naphthyl). Bivalent radicalsformed from substituted benzene derivatives and having the free valencesat ring atoms are named as substituted phenylene radicals. Bivalentradicals derived from univalent polycyclic hydrocarbon radicals whosenames end in “-yl” by removal of one hydrogen atom from the carbon atomwith the free valence are named by adding “-idene” to the name of thecorresponding univalent radical, e.g., a naphthyl group with two pointsof attachment is termed naphthylidene. Whenever it appears herein, anumerical range such as “6 to 10” refers to each integer in the givenrange; e.g., “6 to 10 ring atoms” means that the aryl group may consistof 6 ring atoms, 7 ring atoms, etc., up to and including 10 ring atoms.The term includes monocyclic or fused-ring polycyclic (i.e., rings whichshare adjacent pairs of ring atoms) groups. Unless stated otherwisespecifically in the specification, an aryl moiety is optionallysubstituted by one or more substituents which are independently fluoro,alkyl, heteroalkyl, aryl, heteroaryl, hydroxy, halo, cyano,trifluoromethyl, trifluoromethoxy, —OR^(a), —SR^(a), —OC(O)—R^(a),—N(R^(a))2, —C(O)R^(a), —C(O)OR^(a), —OC(O)N(R^(a))₂, —C(O)N(R^(a))₂,—N(R^(a))C(O)OR^(a), —N(R^(a))C(O)R^(a), —N(R^(a))C(O)N(R^(a))₂,N(R^(a))C(NR^(a))N(R^(a))₂, —N(R^(a))S(O)_(t)R^(a) (where t is 1 or 2),—S(O)_(t)OR^(a) (where t is 1 or 2), —S(O)_(t)N(R^(a))₂ (where t is 1 or2), or PO₃(R^(a))₂ where each R^(a) is independently fluoro, hydrogen,alkyl, fluoroalkyl, aryl, or heteroaryl.

The term “fluoroalkyl” refers to an alkyl radical, as defined above,that is substituted by one or more fluoro radicals, as defined above,for example, trifluoromethyl, difluoromethyl, 2,2,2-trifluoroethyl,1-fluoromethyl-2-fluoroethyl, and the like. The alkyl part of thefluoroalkyl radical may be optionally substituted as defined above foran alkyl group.

The term “fluoroalkoxy” refers to an alkoxy radical, as defined above,that is substituted by one or more fluoro radicals, as defined above,for example, trifluoromethyoxy, difluoromethoxy, 2,2,2-trifluoroethoxy,1-fluoromethyl-2-fluoroethoxy, and the like. The alkyl part of thefluoroalkoxy radical may be optionally substituted as defined above foran alkyl group.

The term “fluoroaryl” refers to an aryl or heteroaryl radical, asdefined above, that is substituted by one or more fluoro radicals, asdefined above, for example, pentafluorobenzene, trifluorobenzene,difluorobenzene, trifluoro-1,10-phenanthroline,pentafluoro-2,2′-bipyridine, and the like.

The term “heteroalkyl” include optionally substituted alkyl radicals andwhich have one or more skeletal chain atoms selected from an atom otherthan carbon, e.g., oxygen, nitrogen, sulfur, phosphorus or combinationsthereof. A numerical range may be given—e.g., C₁-C₄ heteroalkyl whichrefers to the chain length in total, which in this example is 4 atomslong. A heteroalkyl group may be substituted with one or moresubstituents which independently are: alkyl, heteroalkyl, aryl,heteroaryl, hydroxy, halo, cyano, nitro, oxo, thioxo, trimethylsilanyl,—OR^(a), —SR^(a), —OC(O)—R^(a), —N(R^(a))2, —C(O)R^(a), —C(O)OR^(a),—OC(O)N(R^(a))₂, —C(O)N(R^(a))₂, —N(R^(a))C(O)OR^(a),—N(R^(a))C(O)R^(a), —N(R^(a))C(O)N(R^(a))₂, N(R^(a))C(NR^(a))N(R^(a))₂,—N(R^(a))S(O)_(t)R^(a) (where t is 1 or 2), —S(O)_(t)OR^(a) (where t is1 or 2), —S(O)_(t)N(R^(a))₂ (where t is 1 or 2), or PO₃(R^(a))₂ whereeach R^(a) is independently fluoro, hydrogen, alkyl, fluoroalkyl, aryl,or heteroaryl.

“Heteroaryl” or “heteroaromatic” or “HetAr” refers to a 5- to18-membered aromatic radical (e.g., C5-C13 heteroaryl) that includes oneor more ring heteroatoms selected from nitrogen, oxygen and sulfur, andwhich may be a monocyclic, bicyclic, tricyclic or tetracyclic ringsystem. Whenever it appears herein, a numerical range such as “5 to 18”refers to each integer in the given range—e.g., “5 to 18 ring atoms”means that the heteroaryl group may consist of 5 ring atoms, 6 ringatoms, etc., up to and including 18 ring atoms. Bivalent radicalsderived from univalent heteroaryl radicals whose names end in “-yl” byremoval of one hydrogen atom from the atom with the free valence arenamed by adding “-idene” to the name of the corresponding univalentradical—e.g., a pyridyl group with two points of attachment is apyridylidene. A N-containing “heteroaromatic” or “heteroaryl” moietyrefers to an aromatic group in which at least one of the skeletal atomsof the ring is a nitrogen atom. The polycyclic heteroaryl group may befused or non-fused. The heteroatom(s) in the heteroaryl radical areoptionally oxidized. One or more nitrogen atoms, if present, areoptionally quaternized. The heteroaryl may be attached to the rest ofthe molecule through any atom of the ring(s). Unless stated otherwisespecifically in the specification, a heteroaryl moiety is optionallysubstituted by one or more substituents which are independently: alkyl,heteroalkyl, aryl, heteroaryl, hydroxy, halo, cyano, nitro, oxo, thioxo,trimethylsilanyl, —OR^(a), —SR^(a), —OC(O)—R^(a), —N(R^(a))2,—C(O)R^(a), —C(O)OR^(a), —OC(O)N(R^(a))₂, —C(O)N(R^(a))₂,—N(R^(a))C(O)OR^(a), —N(R^(a))C(O)R^(a), —N(R^(a))C(O)N(R^(a))₂,N(R^(a))C(NR^(a))N(R^(a))₂, —N(R^(a))S(O)_(t)R^(a) (where t is 1 or 2),—S(O)_(t)OR^(a) (where t is 1 or 2), —S(O)_(t)N(R^(a))₂ (where t is 1 or2), or PO₃(R^(a))₂ where each R^(a) is independently fluoro, hydrogen,alkyl, fluoroalkyl, aryl, or heteroaryl.

The superior energy density parameters of Cr₂O₇ ²⁻ make it an idealoxidant for use in high-energy, high-power RFBs. The current density ofCr₂O₇ ²⁻ is 14- and 10-fold greater than that of VO₂ ⁺ or Ce⁴⁺,respectively, for equimolar solutions (cf. FIG. 2), while Cr₂O₇ ²⁻ has14- and 17-fold greater volumetric capacity than VO₂ ⁺ or Ce⁴⁺ whencomparing saturated solutions (cf. FIG. 3). While the sodium salt ofCr₂O₇ ²⁻, Na₂Cr₂O₇, has higher solubility than either of these twooxidants (7.1 vs. 3.0 and 2.5 M, respectively), most of the improvementin volumetric capacity is due to Cr₂O₇ ²⁻'s 6 e⁻ reduction, which isirreversible at all cathode catalysts studied by the inventor (glassycarbon, Au, Ag, Pt, Pd, Ni, FIG. 4) and studied in the prior art such asDE914264, or FR7917793 (Pt-group metals (see also Ahmed et al.,“Electrochemical chromic acid regeneration process with fuel-cellelectrode assistance. Part I: Removal of contaminants.” J. Appl.Electrochem. 2001, 31 (12), 1381-1387), poor metal oxides, and othercarbon materials):Cr₂O₇ ²⁻+6e ⁻+14H⁺→2Cr³⁺+7H₂O

In aqueous solutions, Cr₂O₇ ²⁻ exists in equilibrium with chromate, CrO₄²⁻, which can undergo a 3 e⁻ reduction and forms a sodium salt, Na₂CrO₄,with lower solubility (5.4 M). While CrO₄ ²⁻ has lower energy and powerdensity than Cr₂O₇ ²⁻, strongly acidic solutions and higher overall [Cr]greatly favor Cr₂O₇ ²⁻ speciation. Thus conditions of high oxidant andelectrolyte concentration, which are ideal for RFBs, also favor thepredominance of Cr₂O₇ ²⁻:CrO₄ ²⁻+3e ⁻+8H⁺→Cr³⁺+4H₂O2CrO₄ ²⁻+2H⁺⇄Cr₂O₇ ²⁻+H₂O

While Cr₂O₇ ²⁻ represents a path toward RFBs with order-of-magnitudeimprovements in energy and power densities, the key to its successfulimplementation is devising a method to regenerate Cr₂O₇ ²⁻ from Cr³⁺during RFB recharge. This invention proposes to accomplish thishomogeneously via an electron mediator (EM), which undergoeselectrochemically reversible oxidation via heterogeneous reaction at acathode. The resulting series of reactions would follow an EC_(cat)mechanism, where EM is continually regenerated during its oxidationuntil Cr³⁺ is fully recharged to Cr₂O₇ ²⁻:E: EM→EM⁺+1e ⁻C _(cat): 2Cr³⁺+6EM⁺+7H₂O→Cr₂O₇ ²⁻+6EM+14H⁺

Using a 6 e⁻ discharge and a 1 e⁻ recharge should result in asymmetriccharging properties for Cr₂O₇ ²⁻. However, the expected recharge currentwill nonetheless be significantly higher than for the 1 e⁻ EM by itself,since the EC_(cat) mechanism will rapidly replenish EM at the electrodesurface at lower catholyte flow rates.

Thus, one aspect of the present invention is a rechargeable redox flowbattery comprising an electrochemical cell comprising at least onepositive electrode in a positive half-cell and a negative electrode in anegative half-cell, an ion-conducting membrane between the twohalf-cells, wherein the membrane is designed for dual acidic anolyte andcatholyte or designed for an acidic cathodic solution and an alkalineanodic solution; one or more storage tanks, for catholyte and anolyte,one or more pumps to circulate stored catholyte and anolyte through thecathodic and anodic half cells, respectively and at least one anolyteand at least one catholyte. The anolyte comprises at least one fuelcapable of reversible oxidation, at least one electrolyte forconductivity and at least one solvent. The catholyte comprises theCr^(III)/Cr^(VI) redox couple, at least one electrolyte forconductivity, at least one solvent and at least one electrochemicallyreversible electron mediator. Said electron mediator is capable ofhomogeneously oxidizing Cr^(III) to Cr^(VI) using an EC_(cat) mechanismin solution.

In one embodiment, the electron mediator of the redox flow battery asdescribed herein has a standard electrode potential (E⁰) from +0.980 to+1.800 V. Preferably, the standard electrode potential (E⁰) of theelectron mediator is from +1.100 to +1.400V, or from +1.200 to 1.350V.

In some embodiments, the electron mediator of the redox flow battery asdescribed herein, is selected from the group consisting ofCe⁴⁺/Ce³⁺;

Ru(bipy)₃ ³⁺/Ru(bipy)₃ ²⁺, wherein bipy is 2,2′-dipyridine and/or2,2′-dipyridine substituted with one or more substituents chosen fromthe group consisting of fluoro, chloro, bromo, hydroxy, nitro,fluoroalkyl, fluoroalkoxy, fluoroaryl, cyano, alkoxy, carboxy,—OC(═O)R⁰, —SO₃ ²⁻, —SO₂X. or —C(═O)—X, wherein X is hydrogen, C₁-C₆alkyl, —OR¹, fluoroalkyl or —NR²R³, wherein R⁰, R¹, R², and R³ are eachindependently hydrogen, C₁-C₆ alkyl, C₅-C₇ cycloalkyl, perfluoroalkyl,phenyl, substituted phenyl, wherein said phenyl substituents are C₁-C₆alkyl or C₁-C₆ alkoxy, fluoro, chloro, bromo, hydroxy, nitro,fluoroalkyl, fluoroalkoxy or cyano, wherein the optionally substituted2,2′-dipyridine has a more positive reduction potential than E⁰=+1.24 Vvs. NHE, preferably 2,2′-dipyridine is substituted with one or morefluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl, fluoroalkoxy,fluoroaryl, cyano, alkoxy, carboxy substituents, more preferably2,2′-dipyridine is substituted with one or more fluoro, fluoroalkyl,fluoroalkoxy and/or fluoroaryl substituents, most preferably2,2′-dipyridine is substituted with 8 fluoro substituents;

Ru(phen)₃ ³⁺/Ru(phen)₃ ²⁺, wherein phen is 1,10-phenanthroline and/or1,10-phenanthroline substituted with one or more substituents chosenfrom the group consisting of fluoro, chloro, bromo, hydroxy, nitro,fluoroalkyl, fluoroalkoxy, fluoroaryl, cyano, alkoxy, carboxy,—OC(═O)R⁰, —SO₃ ²⁻, —SO₂X. or —C(═O)—X, wherein X is hydrogen, C₁-C₆alkyl, —OR¹, fluoroalkyl or —NR²R³, wherein R⁰, R¹, R², and R³ are eachindependently hydrogen, C₁-C₆ alkyl, C₅-C₇ cycloalkyl, perfluoroalkyl,phenyl, substituted phenyl, wherein said phenyl substituents are C₁-C₆alkyl or C₁-C₆ alkoxy, fluoro, chloro, bromo, hydroxy, nitro,fluoroalkyl, fluoroalkoxy or cyano, wherein the optionally substituted1,10-phenanthroline has a more positive reduction potential thanE⁰=+1.20 V vs. NHE, preferably 1,10-phenanthroline is substituted withone or more fluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl,fluoroalkoxy, fluoroaryl, cyano, alkoxy, carboxy substituents, morepreferably 1,10-phenanthroline is substituted with one or more fluoro,fluoroalkyl, fluoroalkoxy and/or fluoroaryl substituents, mostpreferably 1,10-phenanthroline is substituted with 8 fluorosubstituents;

Fe(phen)₃ ³⁺/Fe(phen)₃ ²⁺; wherein phen is 1,10-phenanthroline and/or1,10-phenanthroline substituted with one or more substituents chosenfrom the group consisting of fluoro, chloro, bromo, hydroxy, nitro,fluoroalkyl, fluoroalkoxy, fluoroaryl, cyano, alkoxy, carboxy,—OC(═O)R⁰, —SO₃ ²⁻, —SO₂X. or —C(═O)—X, wherein X is hydrogen, C₁-C₆alkyl, —OR¹, fluoroalkyl or —NR²R³, wherein R⁰, R¹, R², and R³ are eachindependently hydrogen, C₁-C₆ alkyl, C₅-C₇ cycloalkyl, perfluoroalkyl,phenyl, substituted phenyl, wherein said phenyl substituents are C₁-C₆alkyl or C₁-C₆ alkoxy, fluoro, chloro, bromo, hydroxy, nitro,fluoroalkyl, fluoroalkoxy or cyano, wherein the optionally substituted1,10-phenanthroline has a more positive reduction potential thanE⁰=+1.15 V vs. NHE, preferably 1,10-phenanthroline is substituted withone or more fluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl,fluoroalkoxy, fluoroaryl, cyano, alkoxy, carboxy substituents, morepreferably 1,10-phenanthroline is substituted with one or more fluoro,fluoroalkyl, fluoroalkoxy and/or fluoroaryl substituents, mostpreferably 1,10-phenanthroline is substituted with 8 fluorosubstituents;

Fe(bipy)₃ ³⁺/Fe(bipy)₃ ²⁺; wherein bipy is 2,2′-dipyridine and/or2,2′-dipyridine substituted with one or more substituents chosen fromthe group consisting of fluoro, chloro, bromo, hydroxy, nitro,fluoroalkyl, fluoroalkoxy, fluoroaryl, cyano, alkoxy, carboxy,—OC(═O)R⁰, —SO₃ ²⁻, —SO₂X. or —C(═O)—X, wherein X is hydrogen, C₁-C₆alkyl, —OR¹, fluoroalkyl or —NR²R³, wherein R⁰, R¹, R², and R³ are eachindependently hydrogen, C₁-C₆ alkyl, C₅-C₇ cycloalkyl, perfluoroalkyl,phenyl, substituted phenyl, wherein said phenyl substituents are C₁-C₆alkyl or C₁-C₆ alkoxy, fluoro, chloro, bromo, hydroxy, nitro,fluoroalkyl, fluoroalkoxy or cyano, wherein the optionally substituted2,2′-dipyridine has a more positive reduction potential than E⁰=+1.03 Vvs. NHE, preferably 2,2′-dipyridine is substituted with one or morefluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl, fluoroalkoxy,fluoroaryl, cyano, alkoxy, carboxy substituents, more preferably2,2′-dipyridine is substituted with one or more fluoro, fluoroalkyl,fluoroalkoxy and/or fluoroaryl substituents, most preferably2,2′-dipyridine is substituted with 8 fluoro substituents;

Fe(PR₃)₅ ³⁺/Fe(PR₃)₅ ²⁺, wherein R is chosen from the group consistingof fluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl, fluoroalkoxy,fluoroaryl, cyano, alkoxy, carboxy, —OC(═O)R⁰, —SO₃ ²⁻, —SO₂X. or—C(═O)—X, wherein X is hydrogen, C₁-C₆ alkyl, —OR¹, fluoroalkyl or—NR²R³, wherein R⁰, R¹, R², and R³ are each independently hydrogen,C₁-C₆ alkyl, C₅-C₇ cycloalkyl, fluoroalkyl, phenyl, substituted phenyl,wherein said phenyl substituents are C₁-C₆ alkyl or C₁-C₆ alkoxy,fluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl, fluoroalkoxy, cyano,preferably R is pentafluorobenzene;

Fe(CO)_(x)(PR)_(5-x) ³⁺/Fe(CO)_(x)(PR)_(5-x) ²⁺, wherein x is 1 to 4 andwherein R is chosen from the group consisting of fluoro, chloro, bromo,hydroxy, nitro, fluoroalkyl, fluoroalkoxy, fluoroaryl, cyano, alkoxy,carboxy, —OC(═O)R⁰, —SO₃ ²⁻, —SO₂X. or —C(═O)—X, wherein X is hydrogen,C₁-C₆ alkyl, —OR¹, fluoroalkyl or —NR²R³, wherein R⁰, R¹, R², and R³ areeach independently hydrogen, C₁-C₆ alkyl, C₅-C₇ cycloalkyl, fluoroalkyl,phenyl, substituted phenyl, wherein said phenyl substituents are C₁-C₆alkyl or C₁-C₆ alkoxy, fluoro, chloro, bromo, hydroxy, nitro,fluoroalkyl, fluoroalkoxy, cyano, preferably R is pentafluorobenzene;

Fe(CO)_(x) ³⁺/Fe(CO)_(x) ²⁺, wherein x is 1 to 4;

Cr(η⁶-C₆R_(x)H_(6-x))₂, wherein R is sulfonate (—SO₃ ⁻) and x is 0 to 6;

Cr(CO)₄(P(OC₆R_(x)H_(5-x))₃)₂, wherein R is sulfonate (—SO₃ ⁻) and x=0to 5; Cr(CO)₄(P(OCH₂R)₃)₂, wherein R is sulfonate (—SO₃ ⁻) or hydroxyl(—OH) and mixtures thereof.

In a certain embodiment, the catholyte of the redox flow battery asdescribed herein is a liquid comprising at least one electrochemicallyreversible electron mediator, an electrolyte for conductivity and pHcontrol, including a strong acid with a pK_(a) of 2 or less, preferablyHNO₃, H₂SO₄, HClO₄, H₃PO₄, or mixtures thereof, or simply an electrolytefor conductivity, preferably MClO₄, MNO₃, M₂SO₄, MF, MCl, MBr, or MI,where M=Li, Na, or K, tetra-n-butylammonium X, where X═F, Cl, Br, I, orhexafluorophosphate; and a solvent, selected from the group consistingof water or a nonaqueous solvent, such as acetonitrile,dimethylsulfoxide, dimethylformamide, methanol, ethanol, 1-propanol,isopropanol, ether, diglyme, tetrahydrofuran, glycerol, and mixturesthereof.

In one embodiment, the anolyte of the redox flow battery as describedherein is a liquid comprising a solution of a fuel capable of reversibleoxidation; an electrolyte for conductivity and pH control, including astrong acid with a pK_(a) of 2 or less, preferably HNO₃, H₂SO₄, HClO₄,H₃PO₄, or mixtures thereof, or simply an electrolyte for conductivity,preferably MClO₄, MNO₃, M₂SO₄, MF, MCl, MBr, or MI, where M=Li, Na, orK, tetra-n-butylammonium X, where X═F, Cl, Br, I, orhexafluorophosphate; and a solvent selected from the group consisting ofwater or a nonaqueous solvent, such as acetonitrile, dimethylsulfoxide,dimethylformamide, methanol, ethanol, 1-propanol, isopropanol, ether,diglyme, tetrahydrofuran, glycerol, and mixtures thereof.

In another embodiment, the reversible fuel of the redox flow battery asdescribed herein is a liquid comprising a redox couple selected from thegroup consisting of (1) Zn^(II)/Zn; (2) Fe^(III)/Fe^(II)/Fe; (3)Cu^(II)/Cu^(I)/Cu; (4) H⁺/H₂; (5) V^(III)/V^(II); (6)Cr^(III)/Cr^(II)/Cr; (7) Cr^(III)/Cr^(II); (8) Al^(III)/Al; (9)Zr^(IV)/Zr; (10) Co^(II)/Co; (11) Ni^(II)/Ni; (12) Cd^(II)/Cd; (13)In^(III)/In^(II)/In^(I)/In; (14) Ga^(III)/Ga^(I)/Ga; (15) Sn^(II)/Sn;(16) Sn^(IV)/Sn^(II); (17) Sb^(III)/Sb; (18) Pb^(II)/Pb; (19) Li^(I)/Li;(20) Na^(I)/Na; and/or the oxidized and reduced conjugates of (21)anthraquinone 2,6-disulfonate and mixtures thereof. Preferably, thereversible fuel is selected from the group consisting of Zn^(II)/Zn,Cr^(III)/Cr^(II), Fe^(II)/Fe, V^(III)/V^(II), and Ni^(II)/Ni, and mostpreferably Zn^(II)/Zn⁰.

In one embodiment, the positive electrode of the redox flow battery asdescribed herein comprises at least one cathode catalyst. The cathodecatalyst is selected from the group consisting of glassy carbon,graphite, carbon black, charcoal, oxidatively treated (via plasma,electrochemical, or acid etching) variants of these carbon polymorphs,Au, Pd, Ag, Pt, Ni, Ir, Ru, Rh, alloys of Au, Pd, Ag, Pt, Ni, Ir, Ru, Rhcomprising at least 50% of Au, Pd, Ag, Pt, Ni, Ir, Ru, Rh and mixturesthereof. Preferably the cathode catalyst is selected from the groupconsisting of glassy carbon, graphite, carbon black, charcoal,oxidatively treated variants of these carbon polymorphs, or Au. Mostpreferably the cathode catalyst is Au.

In some embodiments, the redox flow battery as described hereincomprises at least at least two separated cathode catalysts. Preferablythe catalysts are electrically-separated. The first cathode catalyst maybe suitable for Cr₂O₇ ²⁻ reduction (discharge) and the second catalystmay be suitable for high-potential oxidation (recharge) of the mediator.The reduction (discharge) catalyst may be physically inserted andremoved from catholyte, preferably the reduction (discharge) catalyst isinserted and removed via automated actuators. The high-potentialoxidation (recharge) catalyst may likewise, alternatively or inaddition, be physically inserted and removed from the catholyte,preferably the high-potential oxidation (recharge) catalyst is insertedand removed via automated actuators.

In a specific embodiment, the cathode of the redox flow batterydescribed herein is during reduction (discharge) periodically pulsed tolow potentials to remove any precipitates or impurities that may haveaccumulated at the cathode.

In a further aspect, a method for storing electrical energy is provided,which comprises:

-   -   a) circulating an electrolyte through a flow channel of a        positive half-cell in the redox flow battery described herein,    -   b) supplying a current power source to the positive electrode of        said redox flow battery,    -   c) using an electrochemically reversible electron mediator that        is heterogeneously oxidized by the cathode to homogeneously        oxidize Cr³⁺ to Cr₂O₇ ²⁻ without concurrently oxidizing water.

In preferred embodiments of this aspect, the electrolytes and electronmediators are selected from those described in more detail herein above.

In another aspect, the present invention relates to the use of the redoxflow battery as described herein to store electrical energy forgrid-level energy storage, homeowner energy storage, remote locations,firming or load leveling of intermittent renewable electricitygeneration site, preferably wind and solar farms, micro-hydropower,geothermal energy, tidal power, energy arbitrage, portable and/orpersonal electronics, electric vehicles such as ships, submarines,planes, unmanned underwater vehicles (UUVs) or unmanned aerial vehicles(UAVs), military electronics equipment, satellites and other manned orunmanned spacecraft, or other applications where rechargeable RFBs canbe beneficially employed.

Those of skill in the art will realize that many modifications andvariations can be employed without departing from the spirit and scopeof the invention. The present invention is now further illustrated byreference to the following, non-limiting examples.

EXAMPLES

To demonstrate proof-of-concept for the mediated regeneration of Cr₂O₇²⁻, the Ce⁴⁺/Ce³⁺ redox couple was used for the EM, and threedemonstrations were performed to provide evidence for the successfuloperation of the invention.

Example 1

The EC_(cat) mechanism involving the reaction of Cr³⁺ with Ce⁴⁺ toproduce Ce³⁺ was demonstrated via cyclic voltammetry (CV) in thepotential window of +0.2 to +1.5 V vs. Ag/AgCl.

FIG. 5 shows two cases: solutions of (1) 5 mM Ce⁴⁺ and 5 mM Cr₂O₇ ²⁻ and(2) 5 mM Ce⁴⁺, both in aqueous 0.5 M H₂SO₄ at a flat/smooth Auelectrode. In FIG. 5, solutions (1) and (2) show a peak for theoxidation of Ce³⁺ in the anodic (positive-going) scans at +1.44 V, withoxidizing current beginning above +1.35 V, and show a peak for thereduction of Ce⁴⁺ in the cathodic (negative-going) scans at +1.35 V,with net reduction current beginning at +1.40 V. In the CVs, there is noCe³⁺ present in bulk solution, and all detected Ce³⁺ originates fromCe⁴⁺ reduction. Generally, the generated Ce³⁺ will diffuse into bulksolution, but the process is slow and some Ce³⁺ remains in the depletionregion near the electrode surface to be detected in the anodic scans.The Ce³⁺ is generated while the CVs scan below +1.40 V in the cathodicscans and below +1.35 V in the anodic scans.

Since solutions (1) and (2) spend the same amount of time in thepotential range where Ce³⁺ is generated from Ce⁴⁺, the null hypothesisfor lack of interaction between Cr³⁺ and Ce⁴⁺ would be that solutions(1) and (2) should have generated the same amount of Ce³⁺ for detectionin the anodic sweep. However, solution (1) has nearly double the peakcurrent for Ce³⁺ oxidation at +1.44 V. This is attributable to reductionof Ce⁴⁺ by Cr³⁺, the latter of which is generated from Cr₂O₇ ²⁻reduction below +0.9 V in both the anodic and cathodic sweeps.

To preclude the possibility of artifacts in the above result, thecontribution of current from Cr species in the vicinity of Ce³⁺oxidation (>+1.35 V) was assessed using a series of control data. InFIG. 6, the anodic scans of CVs are shown for (1) 5 mM Ce⁴⁺ and 5 mMCr₂O₇ ²⁻; (2) 5 mM Ce⁴⁺; (3) 5 mM Cr₂O₇ ²⁻; and (4) Au in the absence ofCe⁴⁺ or Cr₂O₇ ²⁻, all in 0.5 M H₂SO₄. The anodic scan of Au (4) shows aroughly linear current region at potentials >+1.2 V. Since all of theanodic scans have the same Au background current, Au's linear currentregion was used as an approximate baseline. Anodic scan (5) represents asummation of anodic scans (2) and (3), with one baseline from (4)subtracted. Thus the resulting scan (5) has only one baseline, ratherthan a summation of two baselines from (2) and (3), for bettercomparison to (1). The anodic scan in (5) accounts for only half of theincrease in the Ce³⁺ oxidation peak seen in (1) vs. (2). The additionalcurrent in (1) is clearly attributable to the EC_(cat) process.

Example 2

A discharge/recharge cycle of Cr₂O₇ ²⁻ and Ce⁴⁺ solution wasdemonstrated to fully restore the original Cr₂O₇ ²⁻ reduction current.FIG. 7 shows RDE voltammetry of a solution of 2.5 mM Cr₂O₇ ²⁻ and 2.5 mMCe⁴⁺ in 0.5 M H₂SO₄ at 250 rpm, with Cr₂O₇ ²⁻ reduction occurring atpotentials <+0.9 V. RDE voltammograms are shown for three cases: (1)prior to discharge; (2) after a (non-total) 2.5 C discharge of Cr₂O₇ ²⁻at 4000 rpm; and (3) after 2.5 C recharge of Cr³⁺ at 4000 rpm. The masstransport limited currents in (1) and (3) match, showing fullrestoration of the original Cr₂O₇ ²⁻ concentration.

Example 3

The Cr³⁺ recharge in Demonstration 3 was observed to occur via theexpected asymmetric, EC_(cat) process. FIG. 8.A shows that the 2.5 Cdischarge (negative current and charge) occurred rapidly, taking only 11min, while the recharge in FIG. 8.B required 12 h 30 min. The EC_(cat)recharge of Cr³⁺ via Ce⁴⁺ still occurred faster than simply rechargingCe⁴⁺ from Ce³⁺, as expected for an EC_(cat) reaction. This wasdemonstrated by discharging a solution of 2.5 mM Ce⁴⁺ in 0.5 M H₂SO₄ at4000 rpm for 11 min (FIG. 8.A) and recharging Ce³⁺ to Ce⁴⁺ at the samepotential and rpm used in Demonstration 3 (FIG. 8.B, inset). Current forCe³⁺ oxidation is 2.5 times greater when Cr³⁺ is also present insolution.

These three demonstrations clearly support the functionality of themediated regeneration of Cr₂O₇ ²⁻. Additional aspects of the inventionrelate to further defining the optimal operation and characteristics ofCr₂O₇ ²⁻ RFBs.

In the above demonstrations, EM Ce⁴⁺ is used to demonstrate the EC_(cat)method of Cr₂O₇ ²⁻ recharge. Various embodiments of the invention maymake use of a multitude of EMs with standard electrochemical potentials(E⁰) similar to that of the Cr³⁺/Cr₂O₇ ²⁻ redox couple, specifically E⁰up to 0.250 V negative that of Cr³⁺/Cr₂O₇ ²⁻ (E⁰=+1.23 V vs. NHE, +1.03V vs. Ag/AgCl) and any potential positive that of Cr³⁺/Cr₂O₇ ²⁻ beforethe onset of copious H₂O oxidation at a given electrocatalyst material.For Au and carbon-based electrodes, this includes EMs with E⁰specifically from +0.98 to +1.80 V vs. NHE and +0.78 to +1.60 V vs.Ag/AgCl. Typically, a mediator should be chosen that has E⁰ positive ofthe E⁰ for Cr³⁺/Cr₂O₇ ²⁻ in order to effectively oxidize Cr³⁺ to Cr₂O₇²⁻. However, electrode mediators with E⁰ negative of Cr³⁺/Cr₂O₇ ²⁻ maybe employed since a given mediators E will shift positive of its E⁰ at ahigh ratio of oxidized to reduced mediator, as described by the Nernstequation. During recharge of the Cr₂O₇ ²⁻ RFB, it is expected that 99 to99.995% of the EM will exist in the oxidized form, causing the EM's E⁰to shift positive by 0.120 to 0.250 V. Thus EMs with E⁰ up to 0.250 Vlower than the E⁰ of Cr³⁺/Cr₂O₇ ²⁻ can still effectively oxidize Cr³⁺ toCr₂O₇ ²⁻.

While it would seem that the EM should also have an E⁰ lower than thatof the O₂/H₂O redox couple (E⁰=+1.23 V vs. NHE) to avoid oxidizingaqueous solutions, the vast majority of EMs are not capable of oxidizingH₂O at the E⁰ for O₂/H₂O. The same is true for solid-state electrodematerials. This is because H₂O oxidation to O₂ is a complex reactionrequiring specific catalysts to proceed efficiently. For this reason,neither Au nor most carbon-based electrode materials can performsignificant oxidation of H₂O until potentials of +1.800 V vs. NHE orhigher, allowing a wide potential range where many possible EMs mayoxidize Cr³⁺ to Cr₂O₇ ²⁻.

While prior art, such as Grenet cells, used carbon-based cathodes forCr₂O₇ ²⁻, it is demonstrated that reduction of Cr₂O₇ ²⁻ occurs at farhigher E_(1/2) (higher voltage) at Au cathodes (FIG. 4), and thus Aucathodes are proposed for Cr₂O₇ ²⁻ RFBs. Indeed, Au also outperforms Ag,Pt, Pd, and Ni.

A blocking species (surface oxide, “poison”, or precipitate) thatimpairs Cr₂O₇ ²⁻ reduction seems to develop at higher potentials at theAu cathode. This is evidenced by the high hysteresis between the anodicand cathodic scans in the RDE voltammograms in FIG. 9, with higherE_(1/2) (higher voltage) for Cr₂O₇ ²⁻ reduction observed in the anodicscan. That is, if the Au cathode has temporarily experienced potentialshigher than the onset of Cr₂O₇ ²⁻ reduction, which occurs at thebeginning of a cathodic scan, the E_(1/2) for Cr₂O₇ ²⁻ reduction islower than if the cathode has temporarily experienced potentials lowerthan the onset of Cr₂O₇ ²⁻ reduction. This implies that potentials lowerthan the onset of Cr₂O₇ ²⁻ reduction remove the blocking species andallow higher E_(1/2) (higher voltage). To ensure more efficientdischarge at higher voltage, it is proposed to simply step the electrodepotential to low potentials (specifically 0≤E≤+0.6 V vs. Ag/AgCl) toremove the blocking species. Such potential-step based cleaningtechniques have been developed previously by the author for BH₄ ⁻(Finkelstein et al., “Self-poisoning during BH₄ ⁻ oxidation at Pt andAu, and in situ poison removal procedures for BH₄ ⁻ fuel cells. J. Phys.Chem. C 2013, 117, 1571-1581), and NH₃. These pulse-based techniquesoperate by simulating the change in the electrode potential that occursduring RDE voltammetry that causes the hysteresis between anodic andcathodic scans described above.

Au tends to etch/dissolve at the high potentials necessary for Cr³⁺oxidation. Hence, the present invention also proposes the optional useof dual cathode materials: Au for Cr₂O₇ ²⁻ reduction, and a secondmaterial stable at high potential (Pt, Pd, Ni, carbon, and others) forCr³⁺ oxidation. In some embodiments of the invention, the Au cathodematerial is physically removed from the catholyte during Cr³⁺ oxidationto avoid subjecting it to potentially damaging high potentials.

Various disclosures of the present invention may make use of a number ofreversible anodic half-cell chemistries to complement the describedCr₂O₇ ²⁻ cathodic chemistry. That is, the invention describes a cathodichalf-cell chemistry that may be paired with many different anodicchemistries to form a complete electrochemical cell. It has already beenestablished that the Zn²⁺/Zn anode is compatible with Cr₂O₇ ²⁻. Otherproven and experimental anode chemistries are also likely to operatewell with Cr₂O₇ ²⁻, including Fe³⁺/Fe²⁺, Fe²⁺/Fe, Cu²⁺/Cu⁺, Cu⁺/Cu,H⁺/H₂, V³/V²⁺, Cr²⁺/Cr, Al³⁺/Al, Zr⁴⁺/Zr, Co²⁺/Co, Ni²⁺/Ni, Cd²⁺/Cd,In³⁺/In²⁺, In²⁺/In⁺, In⁺/In, In³⁺/In, In²⁺/In, Ga³⁺/Ga⁺, Ga⁺/Ga,Ga³⁺/Ga, Sn²⁺/Sn, Sn⁴⁺/Sn²⁺, Sb³⁺/Sb, Pb²⁺/Pb, Li⁺/Li, Na⁺/Na, and theanthraquinone 2,6-disulfonate/dihydroxyanthraquinone 2,6-disulfonateredox couple.

The invention claimed is:
 1. A rechargeable redox flow batterycomprising an electrochemical cell comprising: at least one positiveelectrode in a positive half-cell and a negative electrode in a negativehalf-cell, an ion-conducting membrane between the two half-cells,wherein the membrane is designed for dual acidic anolyte and catholyteor designed for an acidic anolyte and an alkaline catholyte; at leasttwo storage tanks for catholyte and anolyte, one or more pumps tocirculate stored catholyte and anolyte through the cathodic and anodichalf cells, respectively, and at least one anolyte and at least onecatholyte; wherein the anolyte comprises at least one fuel capable ofreversible oxidation, at least one electrolyte for conductivity and atleast one solvent; and the catholyte comprises the Cr^(III)/Cr^(VI)redox couple, at least one electrolyte for conductivity, at least onesolvent and at least one electrochemically reversible electron mediator,wherein said electron mediator is capable of homogeneously oxidizingCr^(III) to Cr^(VI) using an EC_(cat) mechanism in solution.
 2. Theredox flow battery according to claim 1, wherein the electron mediatorhas a standard electrode potential (E0) from +0.980 to +1.800 V.
 3. Theredox flow battery according to claim 1, wherein the electron mediatoris selected from the group consisting of: Ce⁴⁺/Ce³⁺; Ru(bipy)₃³⁺/Ru(bipy)₃ ²⁺, wherein bipy is 2,2′-dipyridine and/or 2,2′-dipyridinesubstituted with one or more substituents chosen from the groupconsisting of fluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl,fluoroalkoxy, fluoroaryl, cyano, alkoxy, carboxy, —OC(═O)R⁰, —SO₃ ²⁻,—SO₂X or —C(═O)—X, wherein X is hydrogen, C₁-C₆ alkyl, —OR¹, fluoroalkylor NR²R³, wherein R⁰, R¹, R², and R³ are each independently hydrogen,C₁-C₆ alkyl, C₅-C₇cycloalkyl, fluoroalkyl, phenyl, substituted phenyl,wherein said phenyl substituents are C₁-C₆ alkyl or C₁-C₆ alkoxy,fluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl, fluoroalkoxy orcyano, with the proviso that the optionally substituted 2,2′-dipyridinehas a more positive reduction potential than E⁰=+1.24 V vs. NHE;Ru(phen)₃ ³⁺/Ru(phen)₃ ²⁺, wherein phen is 1,10-phenanthroline and/or1,10-phenanthroline substituted with one or more substituents chosenfrom the group consisting of fluoro, chloro, bromo, hydroxy, nitro,fluoroalkyl, fluoroalkoxy, fluoroaryl, cyano, alkoxy, carboxy,—OC(═O)R⁰, —SO₃ ²⁻, —SO₂X or —C(═O)—X, wherein X is hydrogen, C₁-C₆alkyl, fluoroalkyl or NR²R³, wherein R⁰, R¹, R², and R³ are eachindependently hydrogen, C₁-C₆ alkyl, C₅-C₇cycloalkyl, fluoroalkyl,phenyl, substituted phenyl, wherein said phenyl substituents are C₁-C₆alkyl or C₁-C₆ alkoxy, fluoro, chloro, bromo, hydroxy, nitro,perfluoroalkyl, fluoroalkoxy, cyano, wherein the optionally substituted1,10-phenanthroline has a more positive reduction potential thanE⁰=+1.20 V vs. NHE; Fe(phen)₃ ³⁺/Fe(phen)₃ ²⁺; wherein phen is1,10-phenanthroline and/or 1,10-phenanthroline substituted with one ormore substituents chosen from the group consisting of fluoro, chloro,bromo, hydroxy, nitro, fluoroalkyl, fluoroalkoxy, fluoroaryl, cyano,alkoxy, carboxy, —OC(═O)R⁰, —SO₃ ²⁻, —SO₂X or —C(═O)—X, wherein X ishydrogen, C₁-C₆ alkyl, —OR¹, perfluoroalkyl or —NR²R³, wherein R⁰, R¹,R², and R³ are each independently hydrogen, C₁-C₆ alkyl, C₅-C₇cycloalkyl, fluoroalkyl, phenyl, substituted phenyl, wherein said phenylsubstituents are C₁-C₆ alkyl or C₁-C₆ alkoxy, fluoro, chloro, bromo,hydroxy, nitro, fluoroalkyl, fluoroalkoxy, cyano, wherein the optionallysubstituted 1,10-phenanthroline has a more positive reduction potentialthan E⁰=1.15 V vs. NHE; Fe(bipy)₃ ³⁺/Fe(bipy)₃ ²⁺; wherein bipy is2,2′-dipyridine and/or 2,2′-dipyridine substituted with one or moresubstituents chosen from the group consisting of fluoro, chloro, bromo,hydroxy, nitro, fluoroalkyl, fluoroalkoxy, fluoroaryl, cyano, alkoxy,carboxy, —OC(═O)R⁰, —SO₃ ²⁻, —SO₂X or —C(═O)—X, wherein X is hydrogen,C₁-C₆ alkyl, fluoroalkyl or —NR²R³, wherein R⁰, R¹, R², and R³ are eachindependently hydrogen, C₁-C₆ alkyl, C₅-C₇ cycloalkyl, fluoroalkyl,phenyl, substituted phenyl, wherein said phenyl substituents are C₁-C₆alkyl or C₁-C₆ alkoxy, fluoro, chloro, bromo, hydroxy, nitro,fluoroalkyl, fluoroalkoxy, cyano, wherein the optionally substituted2,2′-dipyridine has a more positive reduction potential than E°=+1.03 Vvs. NHE; Fe(PR₃)₅ ³⁺/Fe(PR₃)₅ ²⁺, wherein R is chosen from the groupconsisting of fluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl,fluoroalkoxy, fluoroaryl, cyano, alkoxy, carboxy, —OC(═O)R⁰, —SO₃ ²⁻,—SO₂X or —C(═O)—X, wherein X is hydrogen, C₁-C₆ alkyl, —OR¹, fluoroalkylor —NR²R³, wherein R⁰, R¹, R², and R³ are each independently hydrogen,C₁-C₆ alkyl, C₅-C₇ cycloalkyl, fluoroalkyl, phenyl, substituted phenyl,wherein said phenyl substituents are C₁-C₆ alkyl or C₁-C₆ alkoxy,fluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl, fluoroalkoxy, cyano,preferably R is pentafluorobenzene; Fe(CO)_(x)(PR)_(5-x)³⁺/Fe(CO)_(x)(PR)_(5-x) ²⁺, wherein R is chosen from the groupconsisting of fluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl,fluoroalkoxy, fluoroaryl, cyano, alkoxy, carboxy, —OC(═O)R⁰, —SO₃ ²⁻,—SO₂X or —C(═O)—X, wherein X is hydrogen, C₁-C₆ alkyl, —OR¹, fluoroalkylor —NR²R³, wherein R⁰, R¹, R², and R³ are each independently hydrogen,C₁-C₆ alkyl, C₅-C₇ cycloalkyl, fluoroalkyl, phenyl, substituted phenyl,wherein said phenyl substituents are C₁-C₆ alkyl or C₁-C₆ alkoxy,fluoro, chloro, bromo, hydroxy, nitro, fluoroalkyl, fluoroalkoxy, cyano,preferably R is pentafluorobenzene; and wherein x is 1 to 4; Fe(CO)_(x)³⁺/Fe(CO)_(x) ²⁺, wherein x is 1 to 4; Cr(η⁶-C₆R_(x)H_(6-x))₂, wherein Ris sulfonate (—SO₃ ⁻) and x is 0 to 6; Cr(CO)₄(P(OC₆R_(x)H_(5-x))₃)₂,wherein R is sulfonate (—SO₃ ⁺) and x=0 to 5; Cr(CO)₄(P(OCH₂R)₃)₂,wherein R is sulfonate (—SO₃ ⁻) or hydroxyl (—OH); and mixtures thereof.4. The redox flow battery according to claim 1, wherein the catholyte isa liquid comprising at least one electrochemically reversible electronmediator, an electrolyte for conductivity and pH control, including astrong acid with a pKa of 2 or less, or simply an electrolyte forconductivity; and a solvent.
 5. The redox flow battery according toclaim 1, wherein the anolyte is a liquid comprising a solution of a fuelcapable of reversible oxidation, an electrolyte for conductivity and pHcontrol, including a strong acid with a pKa of 2 or less, or simply anelectrolyte for conductivity; and a solvent selected from the groupconsisting of water, acetonitrile, dimethylsulfoxide, dimethylformamide,methanol, ethanol, 1-propanol, isopropanol, ether, diglyme,tetrahydrofuran, glycerol, and mixtures thereof.
 6. The redox flowbattery according to claim 1, wherein the reversible fuel is a liquidcomprising a redox couple selected from the group consisting ofZn^(II)/Zn; H⁻/H₂; V^(III)/V^(VI); Cr^(II)/Cr; Cr^(III)/Cr^(II);Al^(III)/Al; Zr^(IV)/Zr; Co^(II)/CO; Ni^(II)Ni; Cd^(II)/Cd;In^(III)/In^(II)/In^(I)/In; Ga^(III)/Ga^(I)/Ga; Sn^(II)/Sn;Sn^(IV)/Sn^(II); sb^(III)/Sb; Pb^(II)/Pb; Li^(I)/Li; Na^(I)/Na; theoxidized conjugates of anthraquinone 2,6-disulfonate, the reducedconjugates of anthraquinone 2,6-disulfonate, and mixtures thereof. 7.The redox flow battery according to claim 1, wherein the positiveelectrode comprises at least one cathode catalyst; wherein said cathodecatalyst is selected from the group consisting of glassy carbon,graphite, carbon black, charcoal, Au, Pd, Ag, Pt, Ni, Ir, Ru, Rh, alloysof Au, Pd, Ag, Pt, Ni, Ir, Ru, Rh comprising at least 50% of Au, Pd, Ag,Pt, Ni, Ir, Ru, Rh and mixtures thereof.
 8. The redox flow batteryaccording to claim 7, comprising at least two separated cathodecatalysts, the catalysts are electrically-separated.
 9. The redox flowbattery according to claim 8, wherein the first cathode catalyst issuitable for Cr₂O₇ ²⁻ reduction (discharge) and the second is suitablefor the high-potential oxidation (recharge) of the mediator.
 10. Theredox flow battery according to claim 9, wherein the first cathodecatalyst is physically insertable and removable from the catholyte. 11.The redox flow battery according to claim 9, wherein the second cathodecatalyst is physically insertable and removable from the catholyte. 12.The redox flow battery according to claim 1, wherein the cathode, duringreduction (discharge), is periodically pulsable to low potentials toremove any precipitates or impurities that may have accumulated at thecathode.
 13. A method to store electrical energy for grid-level energystorage, homeowner energy storage, remote locations, firming or loadleveling of intermittent renewable electricity generation site,micro-hydropower, geothermal energy, tidal power, energy arbitrage,portable and/or personal electronics, electric vehicles such as ships,submarines, planes, unmanned underwater vehicles (UUVs), unmanned aerialvehicles (UAVs), military electronics equipment, satellites and othermanned or unmanned spacecraft, or other applications where rechargeableredox flow batteries can be beneficially employed, comprising the stepof using the rechargeable redox flow battery according to claim
 1. 14. Amethod for storing electrical energy comprising: a) circulating anelectrolyte through a positive half-cell in the redox flow batteryaccording to claim 1, b) supplying a current power source to thepositive electrode of said redox flow battery, c) using the at least oneelectrochemically reversible electron mediator that is heterogeneouslyoxidized by the cathode to homogeneously oxidize Cr³⁺ to Cr₂O₇ ²⁻without concurrently oxidizing water.